Peak detection at a matched-filter output in a radio transmitter X, as shown in FIG. 1, is widely used to determine a time reference point in a radio system having radio transmitter X and a radio receiver Y, where the matched-filter is matched to a synchronization sequence s(i), i=0, . . . , N−1, which has a good auto-correlation property. For example, m-sequences and Gold-sequences (M. K. Simon etc Spread Spectrum Communication Handbook, revised edition, New York, McGraw Hill Inc, 1994) have this desired property. In FIG. 1 a signal generator 10 in a transmitter 12 sends the synchronization sequence over a channel 14 to a receiver 16, a matched filter 18 of which correlates the filter transfer function with the input signal. The peak of the correlation function appearing at the matched-filter output is detected in a peak detection section 19. The transfer function h(i) of the matched-filter is h(i)=s(N−1-i), i=0, . . . , N−1. In ideal situations a peak would appear at time t=t0+N−1 at the matched-filter output of the receiver, where to is the radio propagation time from X to Y. If after a fixed processing time T the same synchronization sequence is transmitted from Y to X, the transmitter X will detect a peak at t=2(t0+N−1)+T at its matched-filter output. Assuming X knows T, it can easily derive the distance from the round-trip delay 2t0 and the speed V0=30 km/s of the electromagnetic wave. Estimation of distance such as this is one example of the application of peak detection methods.
The transmitted synchronization signal on the channel can be expressed as
            s      ⁡              (        t        )              =                  ∑                  i          =          0                          N          -          1                    ⁢                        s          ⁡                      (            i            )                          ⁢                  δ          ⁡                      (                          t              -              i                        )                                ,and the transfer function of the matched-filter as
            h      ⁡              (        t        )              =                  ∑                  i          =          0                          N          -          1                    ⁢                        s          ⁡                      (                          N              -              1              -              i                        )                          ⁢                  δ          ⁡                      (                          t              -              i                        )                                ,where δ(t) is the diracs delta function (impulse) (A V Oppenheim, Digital Signal Processing, Prentice-Hall, Inc, 1975).
Peak detection is usually done by the comparison of the matched-filter output value y(t) to a threshold Φ. Like any linear filter, the matched-filter output is obtained through the convolution y(t)=s(t)*h(t) (John G Proakis, Digital Communication. 3rd Edition McGraw-Hill Inc, 1995). In ideal situations the matched-filter would yield the maximum Pm=Es at t=t0+N−1. where Es is the energy of s(i). In practical systems, the matched-filter is exposed to interference and noise on the channel, therefore, more than one matched-filter output value may exceed Φ. provided that Φ is set so that the probability to detect a correct peak is not zero. Sometimes the peak caused by interference and/or noise can be even higher than the real peak. Sometimes a real peak may not be detected due to suppression by interference and/or noise.
Disadvantages arise with this existing method because the peak may not be found if the signal to noise ratio is too poor. It is an object of the present invention to address the problem of accurate peak detection by providing a more accurate method of peak detection.
According to a first aspect of the present invention a method of detecting a peak/trough in an electrical signal includes sending a synchronization signal from a transmitter for reception by a receiver, wherein the synchronization signal includes a synchronization sequence repeated with a predetermined time interval between repeats and the amplitude of the synchronization sequence is varied between repeats.
The predetermined time interval may be substantially constant.